The objective is to determine whether an increase of fats and animal protein in the diet is associated with an increased risk of breast cancer. This determination is to be made in an ethnic group, the Japanese of California, which exhibits variation from a traditional low fat/animal protein diet to the westernized high fat/animal protein diet. These Japanese women have recently demonstrated a marked increase of incidence of breast cancer. The interview method will be used on a series of diagnosed breast cancer cases and controls matched for age, race, area of residence, and period of hospitalization, with interviews generally conducted after hospital discharge. Place of birth will be controlled in analysis. It is known from a previous interview survey that diet histories vary within both the immigrant and American born groups. Data of the kind proposed here has already been collected on 50 breast cancer cases and 100 matched controls; this proposal is intended to double the size of the existing sample. The necessary cancer notification systems already exist in areas with greatest concentrations of Japanese: Los Angeles County and the five counties of the San Francisco Bay Area.